This invention is related to URD cables, and more particularly to a Underground Residential Distribution (URD) cable where the concentric neutral wires thereof are maintained in a secure electrical connection with a sacrificial anode. The invention also provides a means of preventing corrosion of the neutral wires or URD cable that has been installed underground.
In the present day use of the cable, under certain exposure conditions, corrosion may occur and damage the "tinned" copper concentric neutral wires of Underground Residential Distribution (URD) power cable. The damage can be severe and can develop at a very early date after the URD cables have been installed underground.
The problem is of utmost concern because there have been substantial amounts of cable already buried and greater amounts are scheduled for installation in the future.
The corrosion of the neutral wires may appear as concentrated pitting or the wires may corrode to pencil-point shapes. Sometimes the damage is uniform causing strands to be brittle and crumble at touch. The corroded neutral wires are usually covered with green corrosion products (i.e. oxidized metals). When the wires are first exposed there are active corrosion pits which are filled with a bright red liquid. Shortly after exposure to air this liquid changes to a red-brick color.
It is both impracticable and impossible to attempt to expose every meter of URD cable when looking for corrosion of neutral wires. Fortunately, such as undertaking is not necessary as part of the corrosion process, all metals undergo a change in electrical potential which is measurable and, through experience, is somewhat indicative of physical condition. The corrosion protection as provided by the present invention as well as other means take advantage of the corrosion process itself. When two different metals are buried in the same soil and electrically connected to one another, a corrosion ("galvanic") cell is formed and corrosion current flows, causing the more active metal to be an anode and corrode and the less active or more noble metal to be a cathode and be protected. Sacrificial anodes of zinc or magnesium are generally used for the cathodic protection of the neutral wires.
Up to the present time, several different ways have been used to provide a long-lasting connection between the concentric neutral wires and the anode. However, none of these methods have been efficient or effective. One way which has been used is by use of a mechanical clamp. By this means, a clamp is placed and secured around a corroded area of the neutral wires to provide a contact (i.e. an electrical connection between the neutral wires and the anode connecting wire of the sacrificial anodes). According to this means, the clamp because of its metal fabrication serves as the connection between the concentric neutral wires and the anode connecting wire. That is, the connection wire which makes contact with the anode also makes contact with the common clamp while it is in contact with the neutral wires. This method may seem practical but the clamps have not been effective or beneficial in solving this problem since they will not maintain a low electrical resistance connection for a long period of time.
In another method as described in U.S. Pat. No. 4,080,024 issued to D. H. Kroon, a sheet or sleeve has been used which is placed around the cable where the connection is made between the neutral wires and the connecting wire of the sacrificial anode. This contact or connection is tangential to the neutral wires and is difficult to be maintained over a period of time. Although, this connection means is effective at first, it may become less and less effective with time.
The problems which have accompanied the neutral wires of the URD cable are fully resolved by the means provided by the present invention as described below.